1. Field of the Invention
This invention relates to an ink composition suitable for use in ink-jet printing, and more specifically to an ink composition which, when used in ink-jet printing, has excellent ejection stability under printing conditions and yields a waterfast printed image.
2. Background Art
At the present time, there are several different methods of ink-jet printing, by which a microscopic drop of ink is ejected from a nozzle such that the drop is transported and deposited onto recording media, such as paper. The following are given as examples of methods which can be used for ink-jet printing: static electricity-driven ejection, air pressure-driven ejection, piezoelectric device deformation-driven ejection, and thermal bubble formation-driven ejection.
Conventional inks which are printed using the above methods are well known in the art and include solutions of water-soluble dyes or dispersions of relatively water-insoluble pigments in water or aqueous solutions containing water-soluble organic solvents. The desired qualities for inks to be used in an ink-jet recording system typically include 1) high print quality irrespective of the type of recording media; 2) waterfastness of the printed ink irrespective of the type of recording media; 3) durability of the printed ink with respect to mild abrasion of the printed surface; 4) fast drying of the printed image such that continuously printed pages can be stacked without smearing of the printed images; 5) stability of the ink with respect to temperature extremes; 6) stability of the ink under actual printing conditions such that the nozzles of the print head do not become clogged; 7) the ability of the ink, which has lost solvent(s) as a result of evaporation (dried out), to be redissolved or redispersed when exposed to the ink which has been stored unchanged. This last quality 7) is important in cases in which the ink-jet printer is not used for an extended period of time and the print head must be cleaned by pulsing the head with unchanged ink which is stored in a reservoir, from which evaporation of the solvent components is minimized. The relative importance of the above qualities will depend, of course, on the specific printing application.
It should be noted that, in general for conventional inks, some of the above qualities are not fully compatible with some of the other qualities. For example, conventional inks which are composed of dispersions of relatively insoluble pigments in water or aqueous solutions containing water-soluble organic solvents can be formulated to give extremely high print quality and excellent waterfastness of the printed ink, irrespective of the type of recording media. The waterfastness of such inks is attributed to the inherent waterfastness of the colored pigments. However, because the pigments are dispersed instead of being dissolved in water or aqueous solutions containing water-soluble organic solvents, conventional inks of this type are usually not completely stable with respect to either temperature extremes or to actual printing conditions. Furthermore, in general, it is extremely difficult to formulate inks of this type such that the pigments can be redispersed when dried out ink, for example, in the head, is exposed to the ink which has been stored unchanged, for example, in the head.
As a contrasting example, conventional inks which are composed of water-soluble dyes in water or aqueous solutions containing water-soluble organic solvents can be formulated which yield fast drying printed images and excellent stability against temperature extremes and under actual printing conditions. Because the dyes are water soluble, it is relatively easy to formulate inks of this type in which the dyes can be redissolved when dried out ink is exposed to the ink which has been stored unchanged. However, because the dyes are water soluble, waterfast printed images cannot be obtained on all types of recording media.
As a means of circumventing the above problems with conventional inks which are composed of dispersions of relatively insoluble pigments in water or aqueous solutions containing water-soluble organic solvents, alkali-soluble polymer resins have been used in such ink compositions. Japan Laid-Open publication No.53-114985 discloses the ink composition in which a polymer resin which contains phenolic functionalities is used to form an emulsion with water insoluble pigments or dispersed dyes. The claimed advantage of this ink composition is to yield greater color development than inks which do not contain the polymer resin. The particle size of the polymer-stabilized water-insoluble pigments or dispersed dyes is stated to be less than 3 .mu.m.
Japan Laid-Open Publication No.62-11782 discloses the ink composition in which a polyester resin which contains sulfonic acid functionalities is used to form an emulsion with water insoluble solvent dyes or dispersed dyes. The claimed principal advantage of this ink composition is that the ink composition can yield better waterfastness than inks which do not contain the polyester resin. Information about the particle size of the polymer-stabilized water-insoluble dyes or dispersed dyes is not included in the specification.
Japanese Patent Laid-Open Publication No. 2-248475 discloses the ink composition in which a polymer resin which contains carboxylic acid functionalities is used to form an emulsion with a polymer dispersion which contains either water soluble dyes or water insoluble pigments. The claimed advantage of using that particular alkali soluble polymeric resin is to impart better waterfastness and lightfastness than that of inks which do not contain the polymer. It is claimed in the specification that an alkali-soluble polymer resin containing ink composition is stable for three months at 50.degree. C.
All three of the above examples offer improvements with respect to conventional inks which are composed of dispersions of relatively insoluble pigments in water or aqueous solutions containing water. However, because the pigments or dye containing polymer particles are dispersed instead of being dissolved in water or aqueous solutions containing water soluble organic solvents, such inks are not completely stable against temperature extremes or under actual printing conditions. Settling of the particles over a period of time occurs. Therefore, for these inks, there is still room for improvement. Furthermore, inks of this type cannot be easily redispersed when the ink is dried out and then re-exposed to the ink which has been stored unchanged.
As a means of circumventing some of the problems with conventional inks composed of water or aqueous solutions containing water-soluble organic solvents and water-soluble dyes, ink compositions using alkali-soluble polymer resins have been proposed. Japanese Patent Publication No.40-6581 discloses an ink composition comprising a polymer resin having carboxylic acid functionalities and a water-soluble basic dye. The claimed advantage of this ink composition is that the ink composition can yield an image having better waterfastness than inks which do not contain the polymer resin.
Japanese Patent Laid-Open Publication No. 62-64875 (Japanese Patent Publication No. 5-62633) discloses an ink composition comprising a polymer resin having sulfonic acid functionalities and a water-soluble basic dye. The claimed principal advantage of using that particular alkali soluble polymeric resin is to impart better waterfastness than that of inks which do not contain the polymer.
Japanese Patent Laid-Open No. 5-230413 discloses an ink composition comprising a styrenic polymer resin having carboxylic acid functionalities and a water-soluble basic dye. The average molecular weight of the polymer is in the range of 1500 to 30000 and the acid value of the polymer is in the range of 150 to 300. The claimed principal advantage of using that particular alkali soluble polymeric resin is to impart better waterfastness than that of inks which do not contain the polymer.
The alkali-soluble polymers used in those three examples appear to contribute greatly to the waterfastness of the resulting inks. However, because the dyes are water-soluble basic dyes, the printed images obtained from such inks will not be completely waterfast on all types of recording media. The inherent water solubility of the basic dyes will result in some dissolution of the dye when the printed image is exposed to an excess of water. Therefore, for the above ink composition, there is still room for improvement.